1. Field of the Invention
This invention relates in general to an improvement in a vehicle control system and more specifically to an improvement in a vehicle control system comprising a power steering system for assisting in turning steerable wheels under the action of hydraulic fluid and a vehicle control mechanism for controlling a vehicle operation in response to a steering effort.
2. Description of Prior Art
In order to improve cornering performance in modern automotive vehicles, a steering system including a rear wheel control system has been proposed, which rear wheel control system is disclosed as a compliance steer control system, for example, in U.S. Pat. No. 4,440,254 issued on Apr. 3, 1984 to Shibahata et al.
This compliance steer control system is used in cooperation with a power steering system and includes hydraulic actuators associated with elastomeric insulators interposed between a rear suspension arrangement and a vehicle body. The hydraulic actuators are supplied with hydraulic pressure to be fed to a power cylinder of the power steering system, thus achieving a control to increase or decrease so-called compliance steer of the rear wheels during cornering of the vehicle. This control system arrangement is intended mainly to improve vehicle cornering performance at high vehicle speeds by turning the rear wheels in the same direction as the front wheels upon turning a steering wheel to set vehicle cornering characteristics on the understeer side.
Now, as is well known, in order to provide an appropriate steering feeling to a driver, the conventional power steering system is arranged such that steering assist power developed by the power cylinder is minimized at high vehicle speeds and no steering assist power is developed when the driver's steering effort applied to the steering wheel is very slight. This arrangement provides a so-called non-sensitive region in which no steering assist power is developed. Accordingly, the power cylinder of the power steering system is supplied with low hydraulic pressure at high vehicle speeds and is supplied with no hydraulic pressure when a very slight turning effort is applied to the steering wheel.
However, in the compliance steer control of rear wheels, it is desirable to make a greater compliance steer to turn the rear wheels large amounts during high speed cornering for the purpose of attaining more stable cornering. Additionally, it is also desirable to turn the rear wheels even upon a slight turning angle of the front wheels, i.e., a slight steering effort being applied to the steering wheel when the steering wheel is rotated. In this regard, it has been extremely difficult to control the conventional compliance steering so as to obtain optimum characteristics for cornering performance, by using compliance steer control systems of the above-mentioned type wherein the hydraulic actuators thereof are supplied with the hydraulic pressure fed to the power cylinder of the power steering system. As a result, it becomes necessary to control the hydraulic pressure supplied to the compliance steer control system independently from the hydraulic pressure supplied to the power cylinder of the power steering system. This will be the same in case of controlling body roll of vehicles.